1. Field of the Invention
The present invention relates to a device for dewatering a fibrous web, especially a tissue web. The present invention further relates to a method for dewatering a fibrous web and a machine to produce a fibrous web.
2. Description of the Related Art
Such devices for dewatering a fibrous web are known for the production of voluminous tissue products of high quality. This quality level is also referred to as “premium tissue”. In qualities of this type, a voluminous sheet structure with good absorptive capacity and high water retention capacity is especially important. In producing premium tissue, quality is at the foreground. The production methods are very expensive and energy intensive. The costs of these tissue products are therefore very high.
Document WO2005/075736 A2 describes a machine and a method for the production of premium tissue. After the forming section the fibrous web is dewatered in a dewatering device with a belt press. For this purpose the fibrous web is arranged between a structured fabric and a belt, for example a felt, and is directed over a suction roll. The suction roll is operated with a high vacuum in order to gently dewater the web by means of the hot air flowing through it, whereby dewatering is supported by the belt press. For additional careful dry content increase, an air press or boost dryer is optionally arranged downstream. These devices are very expensive.
An additional possibility of producing premium tissue is offered by the known “through air drying” method (TAD). In this method, large volume flows of hot air or superheated steam are pressed through the fibrous web which is arranged on a structured fabric and directed over a large through-flow cylinder. An expensive air- or steam system is necessary. In the forming section a multitude of vacuum pumps with high energy requirement are additionally required.
In addition to the premium tissue, there is tissue of standard quality. This quality is produced on so-called Crescent tissue machines. These proven tissue machines are of very simple construction, use little energy and are designed for production. However, the quality of the produced fibrous web is clearly below that of premium tissue. This is true respectively also for the prices.
Both qualities are established in regional world markets. With the changes which have occurred over the last few years with regard to raw material and the increased cost of energy, the requirements of the market in regard to quality and prices of tissue papers have also changed. The markets increasingly demand new tissue qualities which, on the one hand are lower than the premium quality, however are clearly higher than the standard quality. The market technology, at the same time should require substantially less energy at lower consumption of high-grade raw materials for the production of the tissue papers.
What is needed on the art is a solution for cost-effective production of tissue papers of intermediate quality. In addition, the tissue machine for the production of tissue papers of intermediate quality is to be sufficiently flexible so that it is possible through rapid modification of the machine to produce premium qualities as well as standard and intermediate qualities.